Grit blasting of rotor housings



ein., U, M949 R, P. MccuLLocH 2,491,771

GRIT BLASTING OF ROTOR HOUSINGS Filed Sept. 27, 1943 2 Sheets-Sheet 1 @en 2u, 3949 R. P. MCCULLQCH ZAQLW GRIT BLASTING OF ROTOR HOUSINGS Filed Sept. 27, 1943 2 Sheets-Sheet 2 nieuwe Dee. zo, 1949 GRIT BLASTING F ROTOR HOUSINGS noten r. McCulloch, Fox Point, wis., assigner,

by mesne assignments, to Borg-Warner Corporation, Chicago, Ill., a corporation of Illinois Application September 27, i943. Serial N o. 504,920

s claims. (ci. 23o- 141) The present invention relates generally to pumps and other fluid displacement apparatus for compressing a fluid. More particularly the improvement contemplated herein relates to superchargers such as employed for internal combustion engines.

In the manufacture of compressors or fluid displacement apparatus that employ lobe-shaped interfltting rotors or impellers (used herein interchangeably) such as those of the Roots blower type, it is the accepted practice to prepare the work surfaces of the housing or compression chamber by machining or dressing these surfaces to a smooth nish with dimensions and shapes to provide minimum clearances between the moving elements in order to develop the highest eiliciency whenthe apparatus is operating under normal working eort. This practice has been found to be objectionable when it is necessary to operate a supercharger or other apparatus at an accelerated or abnormal speed (such for examplefas required for superchargers in aircraft when the latter reaches the more rareed atmospheres of the higher altitudes). The increased speed and higher compressiondevelops heat in the Working parts ofthe apparatus causing expansion, especially in the rapidly spinning blades or lobes, and not infrequently this results in the locking or freezing of the blades, or lobes against the machined surfaces of the compression chamber walls, thus stopping the functioning of the supercharger at a critical time. In the present structure, provision is made to eliminate the above-mentioned objection, and to overcome the liability oi the locking or freezing of the moving parts to the surfaces that are contacted by them.

It is one of the principal objects of this invention to simplify the construction of a supercharger or fluid displacement apparatus such as contemplated herein, and to improve the emciency, operation and dependability of such apparatus.

It is a principal object of this invention to provide a housing wherein the contact or work surfaces are roughened after machining to size, to serve as a means for dressing down a rotor blade or other moving part mounted in said housing. Further, it is an object to provide such rotor blade with one or more ribs or raised lands along the outer periphery, and with chamfered ends or feather edges, adapted to be dressed down or worn away upon contact with the roughened surfaces, and which will also wear down the roughened surfaces. In this connection, it is an object to provideia construction which is preferably provided with a rotor or impeller of comparatively soft material such as aluminum, and which rotor is mounted in a housing having a relatively hard roughened interior working surface, such as a steel, the Whole being assembled to close tolerance before being used, and which is run in on a test block at normal operating speeds, whereby the device will have exactly the operating clearance most needed.

.Another object of this invention is to provide a, fluid displacement device or supercharger wherein the walls of the chamber that houses the rotating blades or lobes have specially prepared surfaces that are of a roughenecl character, and, after the parts have been assembled in their proper arrangement, a certain degree of this roughness is removed by driving the blades or lobes at their normal operating speed to cause the Working portions of said blades or lobes to wear away and also to remove the surplus particles of the roughened faces of the housing walls.

It is also an aim of this invention to provide a supercharger having blasted surfaces on the working faces of the chamber walls, such blasted surfaces being the result of a grit-blasting operation that is performed upon the machined or dressed faces of the housing walls during fabrication of these parts and prior to the :dnal as sembly of the supercharger. It will be appreciated that the Work faces may be roughened by a shot-blasting operation which will provide a surface that is effective for the purpose under certain conditions.

Additional objects, aims, and advantages of' this invention will be apparent to persons: skilled in the art after the construction and operation of the supercharger is understood from the within description.

It is preferred to accomplish the numerous objects of this invention and to practice the same in substantially the manner hereinafter fully described and as more particularly pointed out in the appended claims, reference being made to the accompanying drawings that form a part of this specication.

Fig. 1 is a vertical transverse section of a supercharger or other fluid displacement appa-y ratus, the view being taken on line I--i on Fig. 2;

Fig. 2 is a vertical section of the apparatus shown taken on line 2-2 thereof;

Fig. 3 is an elevational view of the cover plate removed from the housing and viewing the inner surface or work face of said plate; and

Fig. 4 is a fragmentary section taken on line 4-4 on Fig. 2 looking in the direction of the arrows, anci drawn on an enlarged scale.

The drawings are to be understood as being more or less of a schematic character for the purpose of disclosing a typical or preferredform of,

the improvements contemplated herein, and in these drawings like reference characters identify the same parts in the several views.

The structure shown in the drawings embodies a housing, preferably of relatively hard material (such for example as cast iron) comprising semicylindrical end walls d, a'closure plate 6 for the said side wall as shown in Fig. l. to journal the shafts ll and l2 that are mounted for rotation therein, and have their ends that project beyond the outer ends of the bosses it secured to the meshed timing gears i3.

The margins of the side wall l' have a continu-` ous outwardly projecting flange VM that preferably terminates in the vertical plane of the ends of the bearing bosses i@ and a ilanged concave cap l5 is anchored to a lateral lip I6 at the edge of the flange It. This cap, together with the side wall l and the ange it, provides an enclosure for the timing gears in the manner shown in Fig. 1. One of the shafts i Sor l2 projects through the cap I5 where it is provided with a coupling element l1 that connects this shaft to a. suitable prime mover (not shown) whereby the shafts and their rotors are driven at the desired speed. Sultable bushings i are interposed between the shafts Il and I2 and their bearing bosses ill, and

said shafts are provided with annular collars or enlargements I9 beyond the inner end of the bosses I0.

Each of the rotors preferably comprises a relatively soft material (such as aluminum) having a plurality of blades or lobe-shaped member 20 disposed radially from a central hub 2l that is anchored to its respective shaft, the said lobes preferably having the general shape of the blades of a Roots blower. The outer radial extremities of the lobes 20 that follow or move along the inner surfaces of the semi-circular walls 5 of the housing are provided with one 'or more (here shown as two)y parallel ribs or lands 22 that extend the entire length of each blade or lobe in spaced relation to each other.

As shown in- Fig, l, the bore of the rotor hub is increased lndiameter as at 2l, to t over the adjacent inwardly projecting end portion of its bearing boss I0 and a, seal ring 25 surrounds the shaft between the collar i9 and the anged end 26 of the bushing I8. This seal ring 25 is maintained in position by an elastic element such as the spring 21 that is seated at one end in an annular recess 28 in the rotor hub 2| and at its other end presses against the ring 5. The lobes or blades 2li are preferably hollow with suitable reinforcing elements to provide strength.y At their 4margins thev walls of the lobes or blades are undercut or chamfered (herein sometimes referred to as feather edged) as shown at 29 to reduce the thickness of the work edges of the lobes or blades.

After the inner faces of the housing walls i.

l. ne 1 nav'been maenineewtne desired aimensions forl normal operation of the supercharger, these faces are roughened by subjectingv them to a grit-blasting operation of metallic grit, to roughen the portions thereof that are contacted by the lobes or blades. These roughened surfaces resemble flne emery paper, and are indicated by the stipplingli on the drawings. This is accomplished by the application of a blast that contains particles of hard gritty material such as metal filings and the like which produces a multitude of minute indentations and protuberances of a microscopic order on the Asurfaces of the metal that forms the walls. It will thus be apparent that the characteristics of the prepared roughened surfaces produced by grit-blasting are particularly distinguished from plane machined surfaces which have no protuberances. and from unfinished cast surfaces. Furthermore, this gritblasting provides a surface characteristically different from a deep seated wear resistant surface which has been hardened by the development of an area of depressions produced by forcing a tool of relatively small diameter deeply into a heat treated casting.

The parts of the supercharger are then assembled, leaving practically no clearances between the work faces. and thereafter the assembly is coupled to a motor and the parts are run Lat normal operating speed for a period of time to allow the generated heat to expand the parts up to their normal expansion. During this last stage the work edges of the lands or ribs 22 and the feather edged ends, will wear away and likewise will remove any excessively protruding particles of metal from the roughened work faces of the housing so that the desired working clearance is provided between the operating parts of the supercharger. The wearing in will be facilitated by forming the rotors or impellers of relatively soft material as compared to the materials forming the work surfaces of the interior of the housing. An aluminum impeller for example will be rapidly dressed y down by a roughened ferrous v metal work surface. The surfaces may even be a combination of Bakelite or other phenolic or plastic compound with the right metal for the work surfaces of the housing, such as aluminum.

As mentioned above, under certain circumstances shot-blasting might accomplish the purpose herein. For example, on larger superchargers or other devices where the extremely ne adjustment is not so important, shot-blasting might well be used instead of the grit-blasting herein. It is believed that, even for that purpose, grit-blasting is the better, for the reason among others, that shot-blasting has more of a tendency to beat or hammer the metal into small cups than to raise the metal in the way gritblasting does.l While reference is herein made to grit-blasting, and I have found grit-blasting to be much preferable to shot-blasting and other means of raising and roughening the surfaces, still I do not wish to limit myself specifically to grit-blasting'except as I may do so in the claims.

Itis apparent from the above disclosure that by the arrangement herein s et forth it is possible to have only the exact clearance needed for normal operations and thus secure improved 4performance. If an unexpected operation is encountered (which would ordinarily make necessary in any device not incorporating the improvements herein, the leaving of excessive clearance to prevent locking and seizure), therotors will automatically be fitted to size by wearing down in operation, so that there is ordinarily never any more maximum clearance than is absolutely necessary. If the device is never operated at excessive speeds, the loss of efficiency due to leaving additional clearance for expansion is not present in the device herein disclosed.

While this invention has been described in detail in its present preferred form or embodiment. it will be apparent to persons skilled in the art, after understanding the improvement, that various changes and modifications may be made therein without departing from the spirit or scope thereof. It is aimed in the appended claims to cover all such changes and modifications.

I claim:

I. In a fluid displacement apparatus: an enclosing housing formed with a chamber defined by radially extending terminal walls `joined by a generally cylindrical wall, said walls being provided with grit-blasted abrading surfaces; a rtary fluid impeller mounted for rotation in said chamber and having surfaces adapted to normally cooperate in pumping seal relation with said abrading surfaces, said impeller and abrading surfaces initially engaging with no clearance therebetween and said impeller having a hardness less than the hardness of the Walls provided with said abrading surfaces and a coefiicient of expansion higher than that of said walls, whereby upon rotation of said impeller under conditions of normal speed and thermal expansion, said abrading surfaces will automatically function to lap said surfaces of said impeller into normal pumping seal relation with said abrading surfaces, and upon rotation of said impeller under conditions of abnormal speed and thermal exparision said abrading surfaces will automatically function to further lap said surfaces of said impeller and thereby prevent binding or sticking of said impeller in said housing.

2. In combination: a housing for a fluid displacement device containing a chamber for a movable member, said chamber having a gritblasted wall surface providing an abrading worl: face; and a movable member in said housing initially engaging said abrading work face in frictional contact with no clearance therebetween, said movable member having a hardness less than that of said abrading Work face, whereby upon movement of said movable member relative to said abrading work face, a desired working clearance is automatically established between said movable member and said abrading work face by the lapping action of said Work face on said movable member during the movement of said movable member.

3. In combination: a housing for a fluid displacement device, said housing having interior grit-blasted wall surfaces providing abrading work faces; and afrotary impeller in said housing having blade means, said blade means initially engaging said abrading workfaces in frictional contact with no clearance therebetween, said blade means having a hardness less than that of said abrading work faces, whereby upon rotation of said impeller relative to said abrading work faces, a desired working clearance is automatically established between said impeller blade means and said abrading` work faces by the lapping action of said work faces on V said blade means during the rotation of said impeller.

4. A supercharger, comprising: a housing including opposed side wall portions and arcuate end wall portions defining a rotor chamber. said side and end wall portions being provided with initially being in frictional contact with no clearance therebetween and said blades having a hardness less than the hardness of said side and lend wall portions, whereby upon rotation of said rotor members relative to said abrading surfaces, the outermost peripheral portion and end faces of said blades will be automatically dressed down by said abrading surfaces to provide working clearance.

5. In combination: a housing having a rotor chamber provided with grit-blasted interior abrading work faces; and a pair of rotors mounted in said chamber for rotation therein, said rotors each having a plurality of lobes, each lobe being provided with at least one longitudinally extending land on the outer radial extremity thereof, said lobes also being chamfered at the side ends thereof to reduce the area of contact of said ends with said work faces, said lands and lobe ends initially having no clearance relative to said abrading work faces, said lands and lobe ends having a hardness less than that of said abrading work faces, whereby upon rotation of said rotors relative to said abrading work faces a desired working clearance is automatically established between said lands and lobe ends of said rotors, and said abrading work faces, by the lapping action of said work faces while in contact with said rotors during the rotation of said rotors.

6. A supercharger, comprising: a housing including opposed side wall portions and arcuate end Wall portions defining a rotor chamber, said side and end wall portions being provided with particle-blasted abrading surfaces comprising irregularly disposed minute protuberances and indentations of a depth in excess of `that required to provide working clearance for intended normal operation and sufiicient to provide additional working clearance under conditions of :abnormal operation; and a pair of meshing rotor members in said rotor chamber, each of said rotor members being hollow and having walls deiining a plurality of lobes, the outer periphery of each of said lobes being provided with a longitudinally extending narrow land, and the wall at the opposite ends oi' said lobes being chamfered to reduce the thickness thereof, said rotor members and abrading surfaces initially having no clearance therebetween and said rotors having a hardness less than the hardness of said side and end wall portions, whereby upon rotation of said rotor members relative to said abrading surfaces the lands on said lobes and the reduced end walls will be automatically dressed down by said abrading surfaces to provide working clearance.

7. 'Ille method of effecting a desired. working clearance between a housing and movable means in said housing, comprising: providing a housing with a chamber; blasting the interior of said chamber with grit-like particles to provide abrading work faces having minute protuberances and indentations; mounting movable means. of less hardness than said abrading work faces. in said chamber with no clearance between said movable means and said abrading work faces, so that said movable means is initially disposed in intimate frictional contact with said :abrading work faces; and moving said movable means relative to said abrading work faces to dress down the portions thereof vin contact with 4said abrading work faces to thereby provide working clearance between said movable means and said abrading work faces.

8. The method of making a fluid displacing device, comprising: providing a hollow metal housing having dat side walls and arcuate end walls d'elning a rotor chamber; machining the inner l and said abrading work faces so that the lands surfaces of said side and end walls to a desired i size; grit .blasting said machined surfaces to prof vide abrading work laces thereon; mounting a pair of hlacied rotors. oi. less hardness than said housing, in said chamber in intermeshing relation and with no clearance between said rotors and said abraciing Work faces, so that said rotors are initially in intimate frictional Contact with said abrading work faces; and rotating said rotors to dress down the surfaces thereof in contact with said aloradine work faces to provide working clearance between said rotors and loons-` ing. f 9. The method of making a fluid displacing device, comprising: providingv a metallic housing with a rotor chamber; blasting the interior of said chamber with grit-like particles to provide abrading worl; faces; providing a pair o lobed metallic rotors of less hardness and higher coefilcient of expansion. than said abrading work laces; forming lands on the outermost peripheral portion of the lobes of said rotorsg. mounting said rotors in said chamber in intermeshing relation and with. no clearance between said rotors of said rotors are initially disposed in intimate frictional. contact with said abrading work faces; and rotating said rotors to dress down the lands inf contact with said abrading work faces to thereby provide working clearance between said rotors and abrading work faces.

ROBERT P. MCCULLOCH.

REFERENCES CITED The following references are of record in the idle oi this pant.

UNITED STATES PATENTS Number Name Date` 693,609 Kirkwood Feb. 18, 1902 902,702 Wainwright Nov. 3, 1908 971,660 Brewster Oct. 4, 1910 1,404,989 Marcy Jan. 31, 1922 1,563,583 Millspaugh Dec. 1, 1925 1,631,359 Brown June 7, 192'?y 1,784,866 FahrenWa1d- Dec. 16, 1930 2,248,530 Granger July 8, 1941 2,337,903 Lauck -Dec. 28, 1943 2,397,067 Williams Mar. 19, 1946 FOREIGN PATENTS Number Country Date 882 Great Britain 1891 464,475 Great Britain Apr. 16, 1937 

